# Invalid memory access when pp a hash of IO::FileDescriptor =\> SomeObjectInstance

**URL:** <https://forum.crystal-lang.org/t/invalid-memory-access-when-pp-a-hash-of-io-filedescriptor-someobjectinstance/7394>\
**Category:** Help & Support\
**Created:** [November 7, 2024, 6:32pm UTC](https://forum.crystal-lang.org/t/invalid-memory-access-when-pp-a-hash-of-io-filedescriptor-someobjectinstance/7394 "2024-11-07T18:32:39Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![zw963](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/zw963/32/1623_2.png) [@zw963](https://forum.crystal-lang.org/u/zw963)\
**Post date:** [November 7, 2024, 6:32pm UTC](https://forum.crystal-lang.org/t/invalid-memory-access-when-pp-a-hash-of-io-filedescriptor-someobjectinstance/7394/1 "2024-11-07T18:32:39Z")

</div>

I can reproduce the `Invalid memory access` error if add a `pp @readers` or `puts @readers` when uncomment exactly [this line](https://github.com/crystal-china/procodile_cr/blob/refactor_2/src/procodile/supervisor.cr#L253).

following is backtrace, test on Arch linux with Crystal 1.14.0

> **backtrace**
>
> ```auto
> [0x6246abb3d6d6] *Exception::CallStack::print_backtrace:Nil +118 in bin/procodile
> [0x6246abb23726] ~procProc(Int32, Pointer(LibC::SiginfoT), Pointer(Void), Nil) +310 in bin/procodile
> [0x7ceca40691d0] ?? +137355806020048 in /usr/lib/libc.so.6
> [0x6246abc589cb] *Time::Location#lookup<Int64>:Time::Location::Zone +27 in bin/procodile
> [0x6246abc58988] *Time::Location#lookup<Time>:Time::Location::Zone +72 in bin/procodile
> [0x6246abc5439e] *Time#zone:Time::Location::Zone +30 in bin/procodile
> [0x6246abc54361] *Time#offset:Int32 +17 in bin/procodile
> [0x6246abc54321] *Time#offset_seconds:Int64 +17 in bin/procodile
> [0x6246abc55eaf] *Time#year_month_day_day_year:Tuple(Int32, Int32, Int32, Int32) +47 in bin/procodile
> [0x6246abc56606] *Time#year:Int32 +22 in bin/procodile
> [0x6246abd197ee] *Time::Format::Formatter#year:Nil +14 in bin/procodile
> [0x6246abd199aa] *Time::Format::Formatter +10 in bin/procodile
> [0x6246abd1855e] *Time::Format::Formatter +798 in bin/procodile
> [0x6246abd18184] *Time::Format::Formatter +308 in bin/procodile
> [0x6246abd17f0b] *Time::Format#format<Time, IO::FileDescriptor>:Nil +203 in bin/procodile
> [0x6246abc56cb6] *Time#to_s<IO::FileDescriptor, String>:Nil +70 in bin/procodile
> [0x6246abc56b7a] *Time#inspect<IO::FileDescriptor, Bool>:Nil +42 in bin/procodile
> [0x6246abc56b47] *Time#inspect<IO::FileDescriptor>:Nil +39 in bin/procodile
> [0x6246abd119b8] *Procodile::Supervisor +408 in bin/procodile
> [0x6246abd263d1] *Procodile::Instance +1217 in bin/procodile
> [0x6246abd378d1] *Hash(IO::FileDescriptor+, Procodile::Instance) +929 in bin/procodile
> [0x6246abc368bb] *IO::FileDescriptor +27 in bin/procodile
> [0x6246abc3688a] *IO::FileDescriptor +10 in bin/procodile
> [0x6246abb2794d] *puts<Hash(IO::FileDescriptor+, Procodile::Instance)>:Nil +45 in bin/procodile
> [0x6246abd1176a] *Procodile::Supervisor#log_listener_reader:Nil +74 in bin/procodile
> [0x6246abd1382a] *Procodile::Supervisor#watch_for_output:Nil +74 in bin/procodile
> [0x6246abd11e7d] *Procodile::Supervisor#start<Proc(Procodile::Supervisor, Nil)>:NoReturn +461 in bin/procodile
> [0x6246abc6a23d] *Procodile::CLI::start_supervisor<Procodile::Config, Procodile::CliOptions, &Proc(Procodile::Supervisor, Nil)>:Nil +1965 in bin/procodile
> [0x6246abc69920] *Procodile::CLI +2688 in bin/procodile
> [0x6246abb26429] ~proc3Proc(Nil) +9 in bin/procodile
> [0x6246abc6dbca] *Procodile::CLI#dispatch<String>:Nil +202 in bin/procodile
> [0x6246abb1255e] __crystal_main +7070 in bin/procodile
> [0x6246abbc3006] *Crystal::main_user_code<Int32, Pointer(Pointer(UInt8))>:Nil +6 in bin/procodile
> [0x6246abbc2f7a] *Crystal::main<Int32, Pointer(Pointer(UInt8))>:Int32 +58 in bin/procodile
> [0x6246abb20016] main +6 in bin/procodile
> [0x7ceca4051e08] ?? +137355805924872 in /usr/lib/libc.so.6
> [0x7ceca4051ecc] __libc_start_main +140 in /usr/lib/libc.so.6
> [0x6246abb108e5] _start +37 in bin/procodile
> [0x0] ???
> 
> ```

this issue happen when read the hash values of @readers, because if i replace it with: `pp @readers.keys`, no this error, but it happen again when change to `pp @readers.values`, somethings related to Time location cause this?

Can anyone try reproduce it use linux or macOS ?

following is the process:

1. git clone [GitHub - crystal-china/procodile\_cr: 🐊 Run processes in the background (and foreground) on Mac & Linux from a Procfile (for production and/or development environments)](https://github.com/crystal-china/procodile_cr)
2. cd procodile\_cr && git checkout refactor\_2
3. shards build
4. run bin/procodile start -d, it should work as exepcted. like following:

```auto
 ╰──➤ $ bin/procodile start -d
Removed app1.1.pid because process was not active
02:12:17 system | Procodile supervisor started with PID 160490
02:12:17 system | Application root is /home/zw963/Git/procodile_cr
02:12:17 system | Automatic respawning is disabled
02:12:17 system | Reloading configuration
02:12:17 control | Listening at /home/zw963/Git/procodile_cr/pids/procodile.sock
02:12:17 app1.1 | Started with PID 160491
02:12:17 app1.1 | => Fri Nov 8 02:12:17 AM CST 2024 - Hello, World! foo
02:12:18 app1.1 | => Fri Nov 8 02:12:18 AM CST 2024 - Hello, World! foo
02:12:19 app1.1 | => Fri Nov 8 02:12:19 AM CST 2024 - Hello, World! foo
02:12:20 app1.1 | => Fri Nov 8 02:12:20 AM CST 2024 - Hello, World! foo
02:12:21 app1.1 | => Fri Nov 8 02:12:21 AM CST 2024 - Hello, World! foo
02:12:22 app1.1 | => Fri Nov 8 02:12:22 AM CST 2024 - Hello, World! foo

```

1. uncomment [this line](https://github.com/crystal-china/procodile_cr/blob/refactor_2/src/procodile/supervisor.cr#L253), then run following command again to reproduce this issue.
2. shards build && bin/procodile start -d

* * *

In fact there is another random happened issue probably more hard to investigate, but related to this? it’s seem like this issue only happen when `shards build`, but never happen when build with `shards build --release`.

1. Open two console
2. Run `bin/procodile start -d` on first console log should output correctly.
3. Run `bin/procodile restart` on second console if work normal, managed process should be restart in first terminal, you can seeing log output.
4. There is a very high probability second console output `Error: Control server disconnected. data: nil`, with the procodile supervisor process terminated in first console.
5. this issue not happen if build with `--release`.

I trace deeply to code where the issue happen, but, no any clue, because app just exit immediately(in the forked process) without saw any exception backtrace. (rescue on Exception not work), i guess it caused this same `Invalid memory access` issue.

Please help on tracing this bug in the compiler or my code, all method args and return value add the expected type, you can jump to method/class/struct definition use [crystalline](https://github.com/elbywan/crystalline).

---

<div class="post-metadata">

**Author:** ![straight-shoota](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/straight-shoota/32/36_2.png) [@straight-shoota](https://forum.crystal-lang.org/u/straight-shoota)\
**Post date:** [November 7, 2024, 7:56pm UTC](https://forum.crystal-lang.org/t/invalid-memory-access-when-pp-a-hash-of-io-filedescriptor-someobjectinstance/7394/2 "2024-11-07T19:56:55Z")

</div>

This is the source of your issue:

```cr
@started_at = uninitialized Time

```

`uninitialized` literally means the memory is _uninitialized_. The data is random rubbish and you persuade the compiler to accept it contains a valid instance the first time it’s accessed. But this promise is not fulfilled.  
The inspector is trying to access the `Time` instance and print it. So when it dereference uninitilized data as the address of the `Time::Location` instance, you get an invalid memory address.

Don’t use `uninitialized` unless you are aware of the consequences.  
For this use case it’s best to designate an unassigned instance var as `nil` value.

---

<div class="post-metadata">

**Author:** ![zw963](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/zw963/32/1623_2.png) [@zw963](https://forum.crystal-lang.org/u/zw963)\
**Post date:** [November 8, 2024, 5:46am UTC](https://forum.crystal-lang.org/t/invalid-memory-access-when-pp-a-hash-of-io-filedescriptor-someobjectinstance/7394/3 "2024-11-08T05:46:18Z")

</div>

```auto
@started_at = uninitialized Time

```

Good catch, thanks a lot for point it out, I almost forgot I wrote this before.

I really should use `uninitialized` more carefully.

* * *

BTW: the secondary issue still not fixed after fix above bug, even it happens when build with --release too, will keeping investigate more on it, although this is a very small use case.

---

<div class="post-metadata">

**Author:** ![zw963](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/zw963/32/1623_2.png) [@zw963](https://forum.crystal-lang.org/u/zw963)\
**Post date:** [November 8, 2024, 8:39am UTC](https://forum.crystal-lang.org/t/invalid-memory-access-when-pp-a-hash-of-io-filedescriptor-someobjectinstance/7394/4 "2024-11-08T08:39:06Z")

</div>

> [@zw963](#):
>
> In fact there is another random happened issue probably more hard to investigate, but related to this? it’s seem like this issue only happen when `shards build`, but never happen when build with `shards build --release`.
> 
> 1. Open two console
> 2. Run `bin/procodile start -d` on first console log should output correctly.
> 3. Run `bin/procodile restart` on second console if work normal, managed process should be restart in first terminal, you can seeing log output.
> 4. There is a very high probability second console output `Error: Control server disconnected. data: nil`, with the procodile supervisor process terminated in first console.  
> I trace deeply to code where the issue happen, but, no any clue, because app just exit immediately(in the forked process) without saw any exception backtrace. (rescue on Exception not work), i guess it caused this same `Invalid memory access` issue.

~~Still could you figure out the above second issue, i removed all unsafe usage, e.g. `uninitialized Type`, and `not_nil!`, but still could find out why this issue happen.~~

~~I still consider it caused by `Invalid memory access`, following is the evidence:~~

~~1. pull latest code and switch to refactor\_3 branch  
2. if you open file `src/procodile/control_server.cr:33`, you will see code with some pp like following screenshot~~

 ![image](https://canada1.discourse-cdn.com/flex036/uploads/crystal_lang/original/2X/4/42377b979291ed7090f440b6e54b63fab999f7b1.png)

~~3. following above quoted steps of from 1 to 4:~~

```auto
1. Open two console
2. Run `bin/procodile start -d` on first console log should output correctly.
3. Run `bin/procodile restart` on second console if work normal, managed process should be restart in first terminal, you can seeing log output.
4. There is a very high probability second console output `Error: Control server disconnected. data: nil`, with the procodile supervisor process terminated in first console.

```

~~When above issue not happen, all work as expected, you can saw print like:~~

```auto
line # => "restart {\"processes\":null,\"tag\":null}"
# ...
# many log
# ...
response # => "200 [[{\"description\":\"app1.5\",\"pid\":217120,\"respawns\":0,\"status\":\"stopping\",\"running\":false,\"started_at\":1731054436},{\"description\":\"app1.6\",\"pid\":217449,\"respawns\":0,\"status\":\"running\",\"running\":true,\"started_at\":1731054438}],[{\"description\":\"app2.5\",\"pid\":217121,\"respawns\":0,\"status\":\"stopping\",\"running\":false,\"started_at\":1731054436},{\"description\":\"app2.6\",\"pid\":217448,\"respawns\":0,\"status\":\"running\",\"running\":true,\"started_at\":1731054438}],[{\"description\":\"app3.5\",\"pid\":217119,\"respawns\":0,\"status\":\"stopping\",\"running\":false,\"started_at\":1731054436},{\"description\":\"app3.6\",\"pid\":217450,\"respawns\":0,\"status\":\"running\",\"running\":true,\"started_at\":1731054438}]]"

```

~~but once `Control server disconnected` error happen on second panel, we can only saw the `line` print on first console, but no the `response`, like following:~~

```auto
line # => "restart {\"processes\":null,\"tag\":null}"

```

~~That is, somethings wrong happened on following code, but, the rescue Exception never work.~~

```crystal
         pp! line # <== code run until here and exited without any backtrace.
          begin
            response = session.receive_data(line.strip)
          rescue Exception
            STDERR.puts "2"*100
            response = "200 ok"
          end
          pp! response

```

* * *

**EDIT:**

Please ignore above post, I have located the source of this issue.

> <https://github.com/crystal-china/procodile_cr/blob/refactor_3/src/procodile/supervisor.cr#L152-L153>

When add a `sleep 1` or `wg.wait`(or both) here, there’s a chance cause exit app without any backtrace.

If not add `sleep 1` or `wg.wait` here, will cause no log output because @readers never any chance to set correct value, which make listener blocked forever.

Alright, I still have no idea what happens, this issue on happen when run supervisor in the foreground, but never happen when run supervisor in the background.

* * *

So, @straight-shoota , could you please help on explain why this happen? Is there any other situation that could cause Exception can’t be rescue?

---

<div class="post-metadata">

**Author:** ![zw963](https://yyz2.discourse-cdn.com/flex036/user_avatar/forum.crystal-lang.org/zw963/32/1623_2.png) [@zw963](https://forum.crystal-lang.org/u/zw963)\
**Post date:** [November 8, 2024, 3:03pm UTC](https://forum.crystal-lang.org/t/invalid-memory-access-when-pp-a-hash-of-io-filedescriptor-someobjectinstance/7394/5 "2024-11-08T15:03:13Z")

</div>

~~Oops, there is a global rescue in the outermost layer of code make this exception never raised, A very low-level error.~~

~~Thanks, at least, i can confirm, this is not an Invalid Memory Access.~~

I can 100% confirm, this issue really caused by `sleep` or `wg.wait` …